German patent application DE 197 28 814 A1 discloses a cooling system of the generic type. Various temperature levels are to be set in a cooling system for an internal combustion engine of a motor vehicle. The temperature levels to be set here represent the reference input variables for a fan control, which by means of a control program determines the required fan output. The fan output is here determined from the operating parameters of the cooling system, from the reference input variable, and from sets of characteristic curves and characteristics of the fan motor. The operation of the fan is here suspended until the coolant in the cooling system has reached and exceeded a minimum temperature. This is intended to ensure that the internal combustion engine comes up to operating temperature as rapidly as possible and that the fan motor cannot start to exercise a premature cooling effect. Once the fan function is enabled, the control program adjusts the fan output to the set temperature level. Here there are two particular temperature levels of 90° C. and 108° C. to which the fan output has to be adjusted.
The aforementioned cooling system is therefore efficient in as much as it reaches the temperature levels, predetermined as reference input variables, as rapidly as possible. Disadvantages occur, however, when changing from a high temperature level to a low temperature level, since the change of temperature level is predetermined by the change of reference input variable for the output control. In so doing this reference input variable steps down from 108° C. to 95°. The large temperature difference involved in a change of reference input variable from a high value to a low value causes the output control of the fan motor to detect a large temperature difference compared to the current actual temperature, which needs to be corrected as rapidly as possible. This means that the fan motor starts to howl at maximum power. Although this has the advantage that the low temperature level is attained as rapidly as possible, as a rule this is neither desirable nor necessary. The howling of the fan motor is therefore a noise nuisance and leads to unnecessary energy consumption.
This is the point of departure for the invention, the object of the invention being to improve the cooling system so that the fan motor does not start to howl when the set temperature level is changed from a high value to a low value.